Separation of neptunium from uranium hexafluoride containing the same

ABSTRACT

This invention relates to a method of selectively removing neptunium values from a gaseous mixture containing neptunium hexafluoride and uranium hexafluoride by passing the mixture through a bed of pelletized cobaltous fluoride at a temperature in the range 220* F. to 440* F. to effect removal of neptunium by the cobaltous fluoride.

United States Patent 72] Inventors Waldo R. Golliher;

Robert L. Harris; Reynold A. LeDoux, all of Paducah, Ky. 21 App]. No. 806,293 [22] Filed Mar. 11,1969 [4S] Patented Oct. 26, 1971 [73] Assignee The United States of America as represented by the United States Atomic Energy Commission [54] SEPARATION OF NEPTUNIUM FROM URANIUM l-IEXAFLUORIDE CONTAINING THE SAME 3 Claims, No Drawings [52] US. Cl 23/337, 23/326, 23/343, 23/352, 176/14 511 Int. Cl C0lg 43/06 [50] Field of Search 23/326, 337,343,352; 176/10, 13, 14, 16

[56] References Cited UNITED STATES PATENTS 2,838,366 6/1958 Beaufait 23/343 Primary Examiner-Benjamin R. Padgett Assistant Examiner-F. M. Gittes Attorney-Roland A. Anderson ABSTRACT: This invention relates to a method of selectively removing neptunium values from a gaseous mixture containing neptunium hexafluoride and uranium hexafluoride by passing the mixture through a bed of pelletized cobaltous fluoride at a temperature in the range 220 F. to 440 F. to effect removal of neptunium by the cobaltous fluoride.

SEPARATION OF NEPTUNIUM FROM URANIUM HEXAFLUORIDE CONTAINING THE SAME BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

The present invention relates to the processing of uranium hexafluoride and more particularly to a process for selectively removing neptunium values from mixtures of uranium hexafluoride and neptunism hexafluoride.

One of the principal sources of uranium hexafluoride feed for uranium isotope separation by means of gaseous diffusion is derived from the processing of spent nuclear reactor fuels containing irradiated natural uranium. This material is reprocessed to recover plutonium and to separate other transuranium elements as well as fission products from the uranium. Reprocessing is effected by dissolving the irradiated uranium in nitric acid solutions and then extracting the uranium and plutonium values with an organic solvent. The uranium and plutonium are then separated by further solvent extraction or ion exchange cycles. Further details of aqueous separation processes of this type are disclosed in the Reactor Handbook, Vol. II (Fuel Reprocessing), Pgs. 131-184, Second Edition (1961). The purified uranium product is obtained in the form of a uranyl nitrate solution which is converted by subsequent processing to gaseous diffusion feed UF by calcining the uranyl nitrate to form U reducing the UO to UO with hydrogen, reacting the UO with HF to form UF and then finally reacting the UF ,with fluorine.

Although the bulk of the fission products and transuranium elements are removed by this reprocessing scheme, the product uranium still contains significant amounts of neptunium, the quantity of which varies with the irradiation history of the uranium and with the particular separation process involved. Separation of neptunium fluoride from reprocessed UF is desired because it represents a significant health hazard for personnel who operate and repair diffusion cascade equipment. In addition, the presence of neptunium adversely affects the efficiency of the diffusion isotope separation process. Current federal specifications require that the UF feed contain no more than about lpart per million neptunium based on the weight of uranium in the UF feed entering the cascade. It is therefore the principal object of this invention to provide a method of decontamination UF from neptunium which effectively meets this criterion.

SUMMARY OF THE INVENTION The present invention is predicated on the discovery that a porous bed of pelletized cobaltous fluoride will function to selectively reduce neptunium hexafluoride to nonvolatile form or neptunium tetrafluoride at a temperature in the range of 220 F. to 440 F. The neptunium is readily removed from the cobaltous fluoride bed and purified by the following procedure: Fluorine gas is passed through the charged bed at a temperature in the range 500 F. to 700 F. to reconvert the contained neptunium to gaseous neptunium hexafluoride and remove it from the bed.

In order to practice this invention, a porous bed of cobaltous fluoride is required. It should preferably consist of pellets which have sufiicient strength and structural integrity to withstand repeated oxidation and reduction cycles at temperatures ranging from 220 F. to as high as 700 F. Suitable pellets can be prepared by the procedure described in US. Pat. No. 3,372,004, issued Mar. 51968, in the name of Earl W. Richardson et al.

Briefly, the method comprises mixing cobaltous fluoride with water to form a wet agglomerate and then heating the agglomerate to a sintering temperature of l,200 F. in an atmosphere of anhydrous hydrogen fluoride. A satisfactory alternative is to spray the damp agglomerate with water and then air-dry to form suitable pellets. The pellets are sized to produce a batch in the range 1.7 to 2.4 millimeters. A final drying of the sized pellets in anhydrous HF for about lhour at 300 F. serves to reduce the amount of water associated with the pellets and hence eliminates or reduces the possibility for UF to be hydrolyzed to a solid oxyfluoride. The dried pellets have a surface area in the range l0to llsquare meters per gram and a void fraction from 0.75 to 0.85. The treated pellets are loaded into a column to form the required bed. Suitable heating means are provided to heat the cobaltous fluoride bed to a temperature which will maximize selective neptunium removal whereupon mixtures of uranium hexafluoride and neptunium hexafluoride are passed through the column to effect selective removal of neptunium.

Having described the invention in general terms together with a procedure for preparing the active ingredient needed to effect the desired separation, the following examples are provided as exemplary embodiments.

EXAMPLE I Pelletized cobaltous fluoride prepared according to the general procedure described above was loaded into a /a-inch- ID by lZ-inchlong trap. The pellets were dried by heating at 300 F. with a flowing purge of hydrogen fluoride for lhour. Thereafter, mixtures of UF and NpF were passed through the heated trap. Trap temperatures of 440 F., 300 F., and 210 F. were used to detennine the effect of temperature on the separative efi'ect of cobaltous fluoride. Provisions were made to sample the gas stream at the inlet to the trap and at the trap outlet. The results are summarized in table I below.

TABLE I 440 F 300 F. 210 F Weight CoF g 68.3 67.6 71. 5 Pressure, p.s.i.a 14. 7 14. 7 l4. 7 Superficial velocity, 0. 40 0.31 0. 27 Residence time, secs 2. 6 3.2 3.7 Average inlet concentrati Np- 24. 4-40. 4 28 41. 5 Average outlet concentration, ppm.

Np 0. 61-0. 58 0.24 l. 7 Length of run, hrs 9. 755. 25 16 10 Bed analyses:

First inch otmaterial from inlet end:

Concentration of Np, mgJg. 48 22. 2 14.4 Np retained, percent 3 67 27 31 Concentration of U, g./g 0.06 0.06 0.07 Next two inches of material:

Concentration of Np, rngJg. 6 l5. 0 4.1 Np retained, percent 17 37 18 Concentration of U, g./g 0.08 0.070 0.070 Last nine inches of material:

Concentration of Np, mg./g 1.3 3. 3 2. 6 Np retained, percent 16 36 51 Concentration of U, g.lg. 0.09 0. 05 0. 05 Np retained based on inlet and concentration, percent 98 99 96 1 The neptunium concentration is based on uranium.

2 Neptunium inlet concentration increased after 0.75 hours.

3 Neptunium retained is the mg. found in the section divided by the total weight of Np in mg. contained in the 00F: after exposure and represented as a percent.

The results indicate that cobaltous fluoride will selectively remove neptunium at a temperature as low as 210 F., but that the process is more efficient at a temperature in the range 440 F. This is shown by the outlet concentration of neptunium as a function of temperature and the analyses of the cobaltous fluoride after exposure. The difference between 300 F. and 440 F. operation appears insignificant for this trap length when measured by outlet concentration at constant inlet concentrations. However, the bed analyses show that more neptunium is retained in the first inch of the bed operated at 440 F. than for one operated at 300 F. This shows that a substantially higher loading is obtained at 440 F. than at 300 F. Subsequent findings have indicated that no apparent advantage is gained from operating the bed at temperatures higher than about 450 F.

EXAMPLE II The procedure of example I was followed with the exception that the cobaltous fluoride was loaded into a trap Sfeet in length with provisions made to sample the gas stream at the inlet to the trap, after lfoot of cobaltous fluoride, and at the trap outlet. The results of a continuous run are summarized in table II below.

with an outlet concentration of less than 0.2 p.p.m. based on for UF as required by United States Federal Regulations.

What is claimed is:

TABLE II [Loading Studies on Removal of NpFfi from UF -NpF5 with CoFz] Super- Resifieial dence Average Average Average D.F. 1 D.F. Time 1 (hrs) Temp. velocity time Np fed inlet outlet 1 outlet 2 for l-ft for 5-l't. F.) (it/sec.) (secs) (g.) (p.p.m. Np) (p.p.m. Np) (p.p.m. Np) trap trap 300 0. 29 17. 2 3. 7 93. 5 6. 1 0. 12 15 779 300 O. 2O 0. 24 17 0. 67 04 08 25 213 440 O. 24 20 0. 13 9. 8 0. 18 0. O5 54 196 300 0. 50 10 O. 43 9. 0 0. 58 0. O4 16 225 300 1. O 5. O O. 53 13. 3 1. 07 0. 03 12 443 300 0. 5 l0. 0 5. 6 106. 1 3 47. 6 0. 2 265 300 0. 5 10. 0 0. 90 23v 4 8. 0 0v 05 3 468 Total 1 1. 5

1 Designates the number 2 Decontamination factor.

1 Breakthrough occurred in the l-ioot trap during this period.

It should be noted that this run confirmed the previous findings of example I. The selectivity of cobaltous fluoride was evidenced by the removal of l 1.5 grams of neptunium by 340 grams of cobaltous fluoride. The effect of reducing the inlet concentration has only a small effect on the outlet concentration from the 5foot trap. Increasing the temperature to 440 F. shows only a slight improvement. The difference between 0.25 and 1.0 ft/sec. superficial bed velocity shows no significant difference at this loading.

It is concluded from the loading studies presented in examples l and II that a practical system is provided for removal of neptunium from a gaseous stream containing UF and NpF A loading factor in excess of 3.3 percent of the bed is obtainable of hours the system was operated at specified conditions.

1. A method for selectively removing neptunium values from a gaseous mixture of uranium hexafluoride and neptunium fluoride which comprises contacting said gaseous mixture with a bed of cobaltous fluoride pellets at a temperature in the range 200 F. to 440 F. and collecting the resulting exit gas decontaminated from neptunium.

2. The method according to claim 1 in which the uranium hexafluoride gas issuing from the contacted cobaltous fluoride contains less than 2 p.p.m. of neptunium based on the weight of uranium.

3. The method according to claim 1 in which the neptunium is received from the cobaltous fluoride by treatment with fluorine gas at a temperature in the range of500 F. to 700 F.

(5/60) UNITED STATES PATENT OFFICE Patent No. 3,615,267

Dated October 26, 1971 Inventofls) waldo R. Goll'l'her et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column l line 10, "neptunism"' should read neptunium---; line 45,

"decontammation" should read ---'-decontam1nating---; line 66, a comma should be 1nserted after "Mar. 5".

Colomn 2, Table 1, f1 rst line, "CoF should read ---CoF Column 4, line 32, "received" should read ---recovered--.-.

Signed and sealed this 27th day of June 1972.

SEAL) .ttest:

lDwARD M.FLETCHEE;JR.

ROBERT GOTTSCI-LALK .ttesting Officer Commissioner of Patents 

2. The method according to claim 1 in which the uranium hexafluoride gas issuing from the contacted cobaltous fluoride contains less than 2 p.p.m. of neptunium based on the weight of uranium.
 3. The method according to claim 1 in which the neptunium is received from the cobaltous fluoride by treatment with fluorine gas at a temperature in the range of 500* F. to 700* F. 